1. Field of the Invention
The present invention relates to a hollow fiber membrane module used for filtering liquids, and in particular relates to a hollow fiber membrane module, a hollow fiber membrane module unit which utilizes the hollow fiber membrane module, and a purification tank with a hollow fiber membrane module unit installed therein, which are suitable for filtering liquids with high levels of pollutants (especially organic pollutants).
This application is based on Japanese Patent Application No. Hei 8-221567 and Hei 9-217877, the contents of which are incorporated herein by reference.
2. Background Art
Hollow fiber membrane modules have found many applications including the manufacture of sterile water, drinking water and ultra-pure water, and the purification of air, but in recent years various investigations have been conducted into the use of hollow fiber membrane modules for processing highly polluted water, including applications such as the secondary and tertiary processing in sewage treatment plants, solid-liquid separation in purification tanks, solid-liquid separation of suspended solids (ss) from industrial waste water, direct filtration of river water in water purification plants, filtration of tap water for industrial use, and filtration of pool water.
Examples of the use of hollow fiber membrane modules for processing highly polluted water include the hollow fiber membrane modules and hollow fiber membrane module units formed by layering hollow fiber membrane modules, disclosed in Japanese Patent Application, First Publication Nos. Hei 5-261253, Hei 6-340, and Hei 6-342.
FIG. 9 shows an example of a conventional hollow fiber membrane module, wherein a hollow fiber membrane module 50 is constructed of a hollow fiber membrane 3 comprising a plurality of hollow fibers 3a arranged in parallel, and a pair of facing collector pipes 1 fitted to the two extremities of the hollow fiber membrane 3.
The collector pipes 1 have an internal passage, one end of which is closed, while the other end is fitted with a pipe 4 which connects to a suction pump, which is not shown in the figure. Moreover, an aperture 5 is located on the outer wall, and along the length of, each collector pipe 1. Following insertion of the tips of the hollow fiber membrane 3 into the apertures 5, a fixing resin 2 is filled therein, which hardens and closes the apertures, thus fixing and supporting the hollow fiber membrane 3 firmly in place. In this instance, the tips of the hollow fiber membrane 3 are the extremities at either end of the hollow fibers 3a. The two ends of each hollow fiber 3a thus open out into the internal passage inside the collector pipe 1.
To use the hollow fiber membrane module 50 outlined above for suction filtration, the hollow fiber membrane module 50 is positioned inside a tank containing the liquid to be processed, and the pipes 4 are connected to a suction pump.
Activating the suction pump causes the inside of the hollow fibers 3a to develop a negative pressure, thus drawing the liquid to be processed to the hollow fibers 3a, where it permeates through the hollow fibers 3a and is filtered, and the filtrate then passes along the hollow fibers 3a into the internal passage insides the collector pipes 1, through the pipes 4, and is then sucked through the suction pump and discharged.
A plurality of hollow fiber membrane modules 50 with the construction of this type can be arranged inside a tank containing the liquid to be processed, thus forming a hollow fiber membrane module unit. By arranging a plurality of hollow fiber membrane modules 50 inside the tank, the total membrane surface area can be increased, enabling an improvement in processing ability, and in the case of cleaning via methods such as air scrubbing, water and air can move easily between the hollow fiber membranes 3, enabling efficient cleaning of the membrane surface.
However, with the manufacture of this type of hollow fiber membrane module 50, in order to fix the hollow fibers 3a to the collector pipes 1 and then open both ends of the fibers, the two ends of the hollow fibers 3a must first be fixed in place using the fixing resin 2, and the ends of the hollow fiber membrane 3 then opened by inserting a special cutting tool inside the collector pipe 1 and cutting the fixing resin and the ends of the hollow fiber membrane 3. Consequently, it has been impossible to obtain hollow fiber membrane modules with long length collector pipes 1. Furthermore, collector pipes 1 with large outside diameters have also been necessary, which have resulted in a decrease in the pressure tightness of the hollow fiber membrane module 50, and moreover in those instances where a hollow fiber membrane module unit is assembled by layering a plurality of hollow fiber membrane modules 50, the layering ratio per unit volume within the hollow fiber membrane module unit also decreases.
In recent years because the installation of apparatus in purification tanks has fallen greatly in price and the construction period can be shortened, purification tanks made of FRP are becoming widespread.
FIG. 6 shows an example of a purification tank made of FRP, with numeral 70 denoting the purification tank. The purification tank 70 is a cylinder made of FRP, the inside of which provides an effective depth of greater than 1.6 m which can be filled with the water to be processed. The purification tank 70 is buried in the ground with a cylindrical manhole 71 provided on the upper surface of the tank, and the top edge of the manhole opened so that the open portion sits above ground level. The outside diameter of the manhole 71 should be at least 600 mm.
With the spread of purification tanks made of FRP, it might be expected that the use of hollow fiber membrane module units like those described above, which display excellent filtration processing capabilities, as the membrane processing filtration devices inside purification tanks of FRP would be extremely beneficial, but because in the purification tank 70 described above the manhole 71 is the only opening available for the installation or exchange of filtration processing devices, purification of drain water using the hollow fiber membrane module unit described above would require a restriction on the size of the hollow fiber membrane module unit, in order to allow for installation inside the tank. Furthermore, even allowing for the size restriction, a filtration processing device would have to filter at least for a tank for five family (1 m.sup.3 /day), and with conventional hollow fiber membrane module units there would be difficulties with both size and the layering ratio, meaning that installation into purification tanks where there are size restrictions, as in the case of tanks made of FRP, has been problematic.